1. Field of the Invention
The present invention relates to a slip control device of a four-wheel-drive vehicle to prevent slip of wheels by varying the torque transmission distribution on the front wheel side and the rear wheel side via a transfer clutch.
2. Description of the Related Art Statement
Generally, a four-wheel-drive vehicle has an electromagnetic clutch disclosed in, for example, Japanese Unexamined Patent Application Publication No. 7-25258, or a transfer clutch using a hydraulic-driven clutch, and a system capable of varying the torque distribution on front and rear wheels by varying the coupling force of the transfer clutch is known. The system using this transfer clutch performs the slip control to prevent any slip by varying the coupling force of the transfer clutch according to the differential rotation of the wheels when the slip occurs in any one of the front and rear, right and left wheels.
However, in a conventional slip control system, a relatively large dead zone (a zone in which no control is performed) before starting the slip control is provided, and a problem occurs, in that the torque is abruptly changed if the slip control is started over the control starting slip quantity (over the dead zone), and the vehicle behavior is adversely affected.
In order to prevent the slip in this case, it is necessary to control the coupling force of the transfer clutch in the direct coupling direction according to the differential rotation of the front and rear wheels. However, in order to prevent occurrence of any tight cornering brake phenomenon during the turn at a low speed, in other words, any brake phenomenon caused by the differential rotation attributable to the difference in the turning radius between the front and rear wheels, it is necessary to set the coupling force of the transfer clutch in the uncoupling direction when any differential rotation occurs between the front and rear wheels. If the above-described dead zone is simply narrowed, the differential wheel speed caused by the differential locus of the wheels during the turn at a very low speed is mistaken for the slip, the coupling force of the transfer clutch is controlled so as to be increased, and as a result, occurrence of the tight cornering brake phenomenon is promoted.